T.B. Joyce

Transition from electron accumulation to depletion at InGaN surfaces

The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0⩽x⩽1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x=0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0⩽x⩽1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x=0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.

Fabrication of GaN cantilevers on silicon substrates for microelectromechanical devices

The fabrication of free-standing GaN cantilevers on Si(111) is demonstrated, and the growth of III-nitride epilayers on silicon (111) using an AlN buffer layer is characterized. Mechanically releasing GaN structures from Si(111) required a combination of two dry inductively coupled plasma etch processes using Cl2/Ar and CF4/Ar/O2 chemistries, and a potassium hydroxide (KOH) aqueous etch. Scanning transmission electron microscopy reveals a columnar growth habit for the nitrides. Electron energy loss spectroscopy imaging of an AlGaN/GaN interface indicates columnar growth may strongly influence the potential piezoelectric properties of III-nitride cantilever microelectromechanical devices.

A study of surface cross-hatch and misfit dislocation structure in In0.15Ga0.85As/GaAs grown by chemical beam epitaxy

It is well known that a cross-hatch develops on the surface of low-misfit strained semiconductor layers which undergo relaxation by the introduction of arrays of a2〈101〉 misfit dislocations in the interface between the strained layer and substrate. Here we present a study of the detailed structure of these surface striations and their development with thickness in a series of InxGa1 − xAs single layers on (001) GaAs, where x is close to 0.15. Using atomic force microscopy, it is found that the striations are in fact almost triangular ridges with rounded tops separated by V-shaped grooves. They are not slip traces. These ridges are found to be asymmetric in distribution, with those parallel to [110] far higher than those parallel to [110]. The spacing and height of the ridges increases with layer thickness. The structure also becomes more disordered in the case of thicker layers, with ridges running for shorter lengths and having more complex profiles. Using transmission electron microscopy, it is possible to link the ridges to dislocations lying above, and parallel to, the interface which result from repeated operation of multiplication sources.

CBE growth of GaAs/GaAs, GaAs/Si and AlGaAs/GaAs using TEG, AsH3 and amine-alane precursors

The growth of high quality AlGaAs by CBE bas been limited by the high levels of carbon and oxygen contamination. The use of alane based precursors offers a significant reduction in such contamination. We report for the first time the CBE growth of AlxGa1-xAs from triethylgallium, dimethylethylamine-alane and arsine, and compare with. growth from triethylgallium, trimethylamine-alane and arsine. Some preliminary results of work on the CBE growth of GaAs on silicon will also be reported.

Young’s modulus, Poisson’s ratio, and residual stress and strain in (111)-oriented scandium nitride thin films on silicon

Epitaxial scandium nitride films (225nm thick) were grown on silicon by molecular beam epitaxy, using ammonia as a reactive nitrogen source. The main crystallographic orientation of ScN with respect to Si is (111)ScN‖(111)Si and [1–10]ScN‖[0–11]Si; however, some twinning is also present in the films. The films displayed a columnar morphology with rough surfaces, due to low adatom mobility during growth. The strain-free lattice parameter of ScN films grown under optimized conditions was found to be 4.5047±0.0005A, as determined using high-resolution x-ray diffraction (HRXRD). In-plane and out-of-plane strains were subsequently evaluated using HRXRD and were used to determine the Poisson ratio of ScN along the ⟨111⟩ direction, which is found to be 0.188±0.005. Wafer curvature measurements were made and combined with the strain information to determine the average Young’s modulus of the films, which is found to be 270±25GPa. Residual film stresses ranged from −1to1GPa (depending on film growth temperature an...

S-shaped behaviour of the temperature-dependent energy band gap in dilute nitrides

We have investigated the temperature dependence of the band gap energy in GaInNAs, GaNAs and InGaAs quantum wells. In the structures containing nitrogen the well-known S-shaped characteristic was observed at low temperatures. We explain this anomalous temperature behaviour by strong carrier localization in potential fluctuations at low temperatures. In the nitrogen free samples, there was no S-shaped behaviour and the empirical Varshni dependence was followed.

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Abstract Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al x Ga 1− x As layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.

Compositional variation in as-grown GaInNAs/GaAs quantum well structures

Abstract The variation of elemental composition in as-grown GaInNAs/GaAs quantum well structures has been investigated by energy dispersive X-ray analysis of cross-sections using a high resolution scanning transmission electron microscope. The formation of quaternary GaInNAs dot structures is indicated by low temperature photoluminescence measurements and by the correlation of indium and nitrogen distributions. The distributions of arsenic and nitrogen across the well structure suggest the presence of a continuous nitride-like layer formed at the surface of the GaAs buffer layer before the GaInNAs dots. The influence of this nitride-like interlayer on the mechanism of GaInNAs dot formation is discussed.

Structural properties of wurtzitelike ScGaN films grown by NH3-molecular beam epitaxy

Scandium gallium nitride (ScxGa1−xN) alloy films with low Sc concentrations (up to approximately x=0.08) were grown using molecular beam epitaxy with NH3 as a reactive N source, on GaN films that were grown on sapphire using metalorganic vapor phase epitaxy (MOVPE). High-resolution x-ray diffraction and transmission electron microscopy revealed that both the c and the a lattice parameters increased with increasing Sc concentration, as predicted for a wurtzite-structure alloy. As the Sc content increased, the relaxation of the compressive stress in the ScxGa1−xN films occurred mainly via the introduction of additional a-type dislocations, but neither stacking faults nor significant compositional segregation was observed at any composition. A dewetting effect (which increased with increasing Sc content) was observed in MOVPE-grown GaN deposited on top of the ScxGa1−xN films, but the ScxGa1−xN remained compositionally and structurally stable under GaN growth conditions.

Growth of carbon-doped GaAs, AlGaAs and InGaAs by chemical beam epitaxy and the application of in-situ monitoring

Heavily carbon-doped GaAs, AlGaAs and InGaAs were grown by chemical beam epitaxy using CBr 4 as the extrinsic dopant source. SIMS-derived C concentrations of up to 10 21 cm -3 were achieved for GaAs. Comparison of SIMS, Hall and CV data showed 100% activation for C levels as high as 5 x 10 20 cm -3 in GaAs. Oscillations in the reflectance of the growing GaAs films were monitored at 670 nm in real time using the dynamic optical reflectivity (DOR) technique. These oscillations arise from small changes in the refractive index due to carbon doping and are readily observed at doping levels of 2 x 10 19 cm -3 and above. A reduction in the growth rate of heavily C-doped GaAs was observed and this was attributed to etching by Br species associated with the use of CBr 4 . In-situ etching of previously grown GaAs/AlGaAs structures using CBr 4 was also monitored optically, confirming that etching is independent of the supply of TEGa. The growth and doping of ternary compounds, particularly InGaAs, was complicated by the etching of the group III species. InGaAs layers that were carbon-doped were found to be significantly In deficient at doping levels above 10 19 cm -3 ; this was attributed to a higher etch rate for In.